The present invention relates to a hydraulic control device for a torque distribution clutch fitted to a four wheel drive device for a vehicle such as an automobile or the like, and more particularly relates to such a hydraulic control device for such a 4WD torque distribution clutch, said 4WD torque distribution clutch providing selective redistribution of torque between the front wheels of the vehicle and the rear wheels of the vehicle, with said hydraulic control device being capable of providing improved clutch pressure control characteristics.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle can be or are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice, or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is being operated is relatively low. Also, four wheel drive operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics. Therefore, the four wheel drive type of transmission system is becoming more and more popular.
Generally speaking, there are two types of four wheel drive transmission system which are used in practice.
In the first of these two types of four wheel drive transmission system, i.e. in the so called part time type of four wheel drive transmission system, there is provided a two wheel drive/four wheel drive control clutch which selectably either rotationally couples together a drive member for driving the front wheels of the vehicle and a drive member for driving the rear wheels of the vehicle, or alternatively leaves said front wheels drive member and said rear wheels drive member rotationally disengaged from one another. One of said front wheels drive member and said rear wheels drive member is always rotationally driven by the engine of the vehicle via the transmission thereof, and thus, by selective engagement of said two wheel drive/four wheel drive control clutch, the other one of said front wheels drive member and said rear wheels drive member selectably either is rotationally driven by the engine of the vehicle, or is allowed to rotate without being so driven by said engine. Thereby, either four wheel drive vehicle operation or two wheel drive operation is made available for the vehicle. Thus, this two wheel drive/four wheel drive control clutch may be termed a 4WD torque distribution clutch.
On the other hand, in the second of these two types of four wheel drive transmission system, i.e. in the so called full time type of four wheel drive transmission system, there is provided a central differential device, which receives input rotational power from the engine of the vehicle via the transmission thereof, and which always provides said rotational power both to a drive member for driving the front wheels of the vehicle and also to a drive member for driving the rear wheels of the vehicle, with differential action being provided between said front wheels drive member and said rear wheels drive member in a per se known manner. Thus, both said front wheels drive member and also said rear wheels drive member are always rotationally driven by the engine of the vehicle via the transmission thereof and via this central differential device. Such a central or front--rear differential device is typically provided in order to provide a differential action between the front wheels of the vehicle (considered as a pair) and the rear wheels of the vehicle (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possibility of the occurrence of the so called tight corner braking phenomenon created by the difference in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof (and also for various other reasons). And such provision of such a central or front--rear differential device is effective for achieving this result. Further, a central differential control clutch is often per se conventionally provided to this central differential device. According to a control signal such as a control hydraulic pressure which it receives, such a central differential control clutch rotationally couples together to a greater or lesser extent two out of the three rotational power input and output members of this central differential device, so as to allow said central differential device to provide its differential effect as explained above correspondingly to a greater or to a lesser extent. In other words, selectably: when this central differential control clutch is disengaged, it allows said central differential device substantially freely to provide its differential action as explained above; but, when this central differential control clutch is engaged to a greater or to a lesser extent, according to the degree of its engagement, it couples together the rotation of said front wheels drive member and the rotation of said rear wheels drive member, either only somewhat, or absolutely. Accordingly, such a central differential control clutch serves for regulating the distribution of the drive torque produced by the engine of the vehicle between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination). Thereby, either four wheel drive vehicle operation, or two wheel drive operation, or an intermediate form of operation therebetween, is made available for the vehicle. Such a central differential control clutch or central differential action restriction means is typically provided for the following reason. If even one of the vehicle wheels slips, which may well occur especially when the vehicle is being operated upon a bad road surface such as when it is raining, it is snowing, or when the road is muddy, drive power will be lost, whereupon there is the problem that because of the differential effect of the central differential device the drive power of all wheels will be reduced, and the so called trailblazability of the vehicle will be severely deteriorated. In, therefore, a four wheel drive device having a central differential device, such a differential restriction device is typically provided, and is typically operated according to vehicle operational conditions. Again, this central differential control clutch may be termed a 4WD torque distribution clutch.
Various proposals for strategies for qualitative control of the engagement state of such a 4WD torque distribution clutch, according to various operational conditions, have been made in various prior arts. And proposals for controlling such a 4WD torque distribution clutch have been put forward, for example, in Japanese Patent Application Laying Open Publication Ser. No. 58-20521 (1983), in Japanese Patent Application Laying Open Publication Ser. No. 60-176827 (1985), and in Japanese Patent Application Laying Open Publication Ser. No. 62-139722 (1987), all of which were filed by an applicant the same as the entity owed duty of assignment of the present patent application, and none of which is it hereby intended to admit as prior art to the present patent application except to the extent in any case mandated by applicable law. Such a 4WD torque distribution clutch is typically of a hydraulic type which is powered by a hydraulically operated servo device, so that the engagement pressure of said 4WD torque distribution clutch, i.e. the maximum amount of torque that said 4WD torque distribution clutch can transmit, which defines the amount of torque distribution which said 4WD torque distribution clutch can provide between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination), is regulated by the magnitude of an actuating hydraulic fluid pressure. And such an actuating hydraulic fluid pressure is typically provided by a control system such as a hybrid electrical/hydraulic control system which may include a microcomputer. In more detail, typically such an actuating hydraulic fluid pressure is typically provided by a control system which operates by duty ratio control or by dither control, the control being performed by such a microcomputer.
In more detail, controlling such an actuating hydraulic fluid pressure by duty ratio control is done by supplying a pulsed electrical signal of substantially constant pulse frequency to a solenoid or the like of an on/off type of electromagnetic fluid switching valve, and by varying the duty ratio of said pulsed electrical signal according to the pressure value which it is required that said valve should output. On the other hand, controlling such an actuating hydraulic fluid pressure by dither control is done by supplying an electrical signal such as a current signal to a linear type of electromagnetic fluid switching valve, thus (for example) controlling the opening amount of a drain port thereof, thereby varying the pressure value which said valve is outputting, and by causing this electrical signal somewhat to pulsate, so that the valve element of said valve constantly oscillates with a relatively very small amplitude about its desired position. Thereby, sticking of this valve element is prevented, and the responsiveness of the valve is enhanced.
With either of these control methods--either duty ratio control or dither control--if the line hydraulic fluid pressure of the transmission, which typically increases along with an increase in the load on the engine of the vehicle, is used directly as the source of hydraulic fluid pressure which is modulated by the electromagnetic fluid switching valve in order to provide the actuating hydraulic fluid pressure for the 4WD torque distribution clutch, then, although the hydraulic circuit is thereby rendered relatively simple, the problem arises that, since this base pressure which is modulated for controlling the 4WD torque distribution clutch varies, then, in the exemplary case of duty ratio control, even in the case of a fixed duty ratio of the controlling pulsed electrical signal, the actuating hydraulic fluid pressure for the 4WD torque distribution clutch varies along with these line pressure variations. Accordingly, the duty ratio of the controlling pulsed electrical signal is required to be compensated, in order to allow for the current value of the line pressure, and thereby the control of this duty factor becomes very complicated, and it becomes difficult to have accurate control of the actuating hydraulic fluid pressure for the 4WD torque distribution clutch.
In other words, when controlling the actuating hydraulic fluid pressure for the 4WD torque distribution clutch by duty ratio control of the pulsed electrical signal which is supplied to the electromagnetic fluid switching valve, the ratio of the servo pressure supplied to the duty ratio of said pulsed control signal will vary depending upon the current value of the line pressure of the transmission, and, since also the pressure which is being supplied to said electromagnetic valve is constantly varying, the pressure adjustment characteristics provided are not stabilized, and thereby the accuracy of the control of said actuating hydraulic fluid pressure is not good, and moreover it is required that the electromagnetic fluid switching valve should be of a high pressure type so that it can definitely tolerate the maximum conceivable value of line pressure, which means that the electromagnetic fluid switching valve is required to be relatively complicated and relatively large.
Another problem that has occurred with respect to the prior art and with respect to the constructions detailed above is that, with either of these control methods for the electromagnetic fluid switching valve--either duty ratio control or dither control--either as the valve element opens and closes, or as the valve element wobbles to and fro about its desired position, as the case may be, fine fluctuations or fluttering in the output pressure of the electromagnetic fluid switching valve are caused, and this creates a wobbling or fluctuation in the hydraulic fluid pressure in the servo chamber of the 4WD torque distribution clutch, which causes the engagement pressure of said clutch and hence the torque transmission capacity thereof to be not properly stabilized, and accordingly the torque distribution between the front wheels of the vehicle and the rear wheels of the vehicle fluctuates finely but at high frequency, which deteriorates the drivability and the operational feeling of the vehicle, as well as increasing tire wear and reducing roadability.